Spectral Transitions of an Ultraluminous X-Ray Source, NGC 2403 Source 3

Abstract

Abstract A Suzaku observation of an ultraluminous X-ray source, NGC 2403 Source 3, performed on 2006 March 16–17, is reported. The Suzaku XIS spectrum of Source 3 was described as multi-color black-body-like emission from an optically thick accretion disk. The innermost disk temperature and radius were measured to be $T_{\rm in}$$=$ 1.08$_{-0.03}^{+0.02}$keV and $R_{\rm in}$$=$ 122.1$_{-6.8}^{+7.7} \ \alpha^{1/2}$km, respectively, where $\alpha$$=$ (cos 60$^\circ /$cos $i$) with $i$ being the disk inclination. Its bolometric luminosity was estimated to be $L_{\rm bol}$$=$ 1.82 $\times$ 10$^{39}\alpha $ergs$^{-1}$. Archival Chandra and XMM-Newton data of the source were analyzed. In almost all observations, the source showed multi-color black-body-like X-ray spectra with parameters similar to those of the Suzaku observation. In only one Chandra observation, however, Source 3 exhibited a power-law-like spectrum, with a photon index of $\Gamma$$=$ 2.37$\pm$0.08, when it was fainter by $\sim$15% than in the Suzaku observation. This behavior is explained in terms of a transition between a slim-disk state and “very high” states, both found in Galactic black hole binaries, when their luminosity apporached the Eddington limit. These results are utilized to argue that ultraluminous X-ray sources generally have significantly higher masses than ordinary stellar-mass black holes.